Modern cardiac pacing systems have incorporated substantial capability for detecting and dealing with various arrhythmias. Of particular importance are atrial arrhythmias such as atrial fibrillation (AF), which may lead to serious complications. Atrial fibrillation is manifested as an irregular disorganized activity of the heart, and in the absence of complete AV block, the ventricular response is irregular and random. The irregularity of the resulting cardiac rhythm adversely affects the contractile performance of the heart. It is a source of considerable morbidity and mortality; AF is the leading cause of embolic stroke. As used hereinafter, the term atrial fibrillation, or AF, refers broadly to the class of dangerous atrial arrhythmias, during episodes of which it is desired to inhibit conduction of most of the atrial signals to the ventricles. Pacemakers have attempted to deal with such arrhythmias by simply switching into an asynchronous mode, such that ventricular pacing does not try to track the dangerous atrial excitations. However, with ordinary asynchronous ventricular pacing and continued conduction of the atrial signals through the AV node, a certain percentage of the atrial signals will get through to the ventricle and thus cause chaotic spontaneous ventricular contractions and paced contractions, resulting in an undesirable cardiac condition. Patients with paroxysmal or chronic AF and intact AV conduction who are highly symptomatic and drug refractory are presently candidates for His ablation. This is, of course, a procedure which stops conduction of all atrial signals to the ventricle permanently. The result is that the ventricle needs to be paced permanently even though the atrium contracts normally most of the time.
Another technique that is in use is that of delivering a cardioversion shock to the patient's heart. This can be done during general anesthesia, which of course is impractical for a patient who has repeated and rather long-occurring episodes. Such a patient would also be a candidate for an implantable cardioverter device. However, such devices are very expensive, and the shocks are not welcome to the patient, i.e., they may be painful. Further, if the episodes occur too frequently, these devices have a limited lifetime due to the energy expenditure of each shock.
Another approach known in the literature is to cool the atrium, thereby slowing conduction in the atrial tissue to the point of terminating the atrial fibrillation. See Abstract, Scaglione et al, PACE, Vol. 16, p 880, April 1993, Part II. In this approach, the entire atrium is cooled by introduction of a bolus of cold saline solution. See also U.S. Pat. No. 5,876,422, issued Mar. 2, 1999, showing a system for Peltier cooling of the AV node during which the ventricle must be paced asynchronously for the duration of the AF episode.
Another approach to the problem is for the pacemaker to respond by aggressively pacing at a higher, but more stable rate. See, for example, U.S. Pat. No. 5,480,413. See also U.S. Pat. No. 5,792,193, which smooths the ventricular rate by an algorithm that allows some spontaneous ventricular contractions, and delivers some pace pulses which overdrive the spontaneous rate.
However, there remains a substantial need for an improved system and technique for effectively regulating the ventricular rate until the atrium can return on its own to a normal sinus rhythm, and without requiring a high ventricular rate so that the ventricle be paced asynchronously.